I can't find where I saw post saying that it is against the rules, but let me explain.
If I have 1200+ dragons and I'll say "offer anything I don't have", will you check my whole scroll to see what I need? No, you won't.
You should say what you want like this: looking for a 3G Silver x (m) Chrono checker / looking for 2-4 any CB hatchlings / offer a dummy / bloodswap and so on. You can't say anything like "offer anything I don't have" or "[nick] offer again". Also, you can't put any off-site (Edit: thanks @HeatherMarie for correcting!) links in the box or say "pm me on forum".
Fun fact: you can't even say "offer 2G from val '09 alt" or anything similiar to it, because only two people on whole DC have this alt. I think Trading Hub should get guidelines or something. For now, you can get banned for a lot of things.
I'll search for this post, but it'll take me a while to scroll through 1000+ replies on Trading Hub thread.